1) Field of the Invention
This invention relates to a triple-intake-valve internal combustion engine, namely, to an internal combustion engine having three intake valves per combustion chamber.
2) Description of the Related Art
Five-valve engines having three intake valves on an intake side and two exhaust valves on an exhaust side are already known. One example of such conventional five-valve engines is illustrated in FIGS. 10 and 11. In these examples, three intake ports 2,3,4 and two exhaust ports 5,6 open at their respective ends through a ceiling of each combustion chamber 1 of the pentroof type. These ports are opened and closed by unillustrated intake valves and exhaust valves at appropriate timing. The intake ports 2,3,4 are branched near the combustion chamber 1, and a fuel injector 8 is arranged in a branching portion 7 where a common intake port branches into the individual intake ports 2,3,4. The fuel injector 8 is constructed such that fuel can be injected in substantially equal amounts into the three intake ports 2,3,4.
The fuel injector 8 is controlled in such a way that an air-fuel mixture to be fed therethrough has substantially a stoichiometric air/fuel ratio. Flows 2A,3A,4A of the air-fuel mixture drawn through the respective intake ports 2,3,4 flow in the form of swirls throughout the combustion chamber 1 as shown in FIG. 10, whereby the flows 2A,3A,4A are agitated. Described in more detail, the intake ports 2,3,4 are branched near the combustion chamber 1 so that the directions of the flows 2A,3A,4A are not parallel with each other. As a result, the flows 2A,3A,4A are fully agitated in the combustion chamber 1. According to such an intake system structure, ideal instantaneous combustion can be achieved by the agitating effects of the triple tumble flows, thereby bringing about the characteristic advantages that high combustion efficiency and hence high output can be obtained.
Recently, there is urgent necessity to develop an engine of high fuel economy from the viewpoint of energy saving. The engine shown in FIG. 10, however, still provides insufficient fuel economy although it features high output owing to high combustion efficiency.
In the meantime, an engine permitting combustion of fuel at an air-fuel ratio leaner than the stoichiometric air-fuel ratio has been disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 3-23314. The engine is provided with at least two intake ports. Fuel is injected into only one of the intake ports to use it as an air-fuel mixture feed passage and the other intake port is used as an air feed passage, whereby a tumble flow of the air-fuel mixture and a tumble flow of air are formed in a combustion chamber under barrel-stratification. The tumble flow of the air-fuel mixture is then ignited. Although this patent publication also discloses one embodiment of an engine having three intake ports, the intake port located on one side is used as a feed passage for an air-fuel mixture. When a tumble flow of the air-fuel mixture in the combustion chamber is ignited to burn, one side of the tumble flow is an inner wall of the combustion chamber so that the efficiency of oxygen utilization is impaired. Further, the cross-sectional shape of each intake port is circular so that formation of the intake port into a straight shape with a view toward a stronger tumble flow in the combustion chamber results in the crossing of the intake port at an acute angle with its opening to the combustion chamber. As a consequence, the cross-sectional flow area of each intake port is substantially reduced. In other words, a stronger tumble flow results in a reduction in the maximum flow rate of each intake port having a circular cross-section, whereby the performance of the engine at full power is lowered.